In this paper, we would like to quantify factors that contribute to the cacheability decision of Web objects. Our study is based on a comprehensive study of object attributes that are related to the HTTP protocol transfer, proxy preference, content availability, freshness revalidation as well as the inter-relationship and dependency among them. Furthermore, the relative importance of these factors towards the final cacheability decision of Web objects is also investigated. This study is important because it provides hints on how to optimize Web caching performance